1. Field of the Invention
The present invention relates to continuous wave ranging systems and, in particular, to a signal processor for use therein.
2. Description of the Prior Art
In continuous wave (CW) radar ranging systems, a frequency modulated interrogation signal is transmitted toward a target and is reflected therefrom back to the interrogating unit. The reflected signal is received by the interrogating unit and is mixed with a sample of the interrogation signal, and filtered to obtain a difference signal. In a cooperative harmonic system the return signal is a given harmonic of the frequency of the interrogation signal. The reflected signal is therefore mixed with a suitably frequency multiplied sample of the interrogation signal.
The finite distance or range between the interrogating unit and the target introduces a round trip delay .tau. between the return signal and the instantaneous interrogation signal sample. EQU .tau. = 2R/C (1)
where R is the range and C is the velocity of light. Hence, since the interrogation signal is frequency modulated with a given modulation waveform, the reflected signal as received at the interrogating unit is delayed in time and hence shifted in frequency from the instantaneous interrogation signal by an amount proportional to the range. For example, where a triangular waveform having a peak value of .DELTA.F and a period of 1/f.sub.m is used to frequency modulate the interrogation signal, the frequency shift or difference frequency f.sub.R, as generated by a suitably filtered mixer, equal to the time derivative of the frequency of the interrogation signal times the round trip time delay, is: ##EQU1## Thus, the range between the target and the interrogating station may be computed by a measurement of frequency shift f.sub.R.
Conventional processors measure the difference frequency by counting the number of zero crossings that occur within a fixed time interval. More specifically, the difference frequency is applied to a counting circuit which develops a signal that is proportional to the rate of zero crossings.
However, the difference frequency waveform is periodic in the frequency (f.sub.m) of the modulation waveform. The average measured frequency as determined by counting zero crossings in a fixed time interval, therefore, must be a multiple of f.sub.m. Thus, the measured frequency is quantized and accordingly, so is the range. For the above exemplary triangular modulation waveform, the quantization step in the range, .DELTA.R, is equal to EQU .DELTA.R = (C/8.DELTA.f) (3)
Conventional systems provide for minimization of quantization by choosing a maximum frequency deviation .DELTA.F of the frequency modulation as large as possible. In practice, however, inherent bandwidth limitations on mixing circuits, harmonic generators when applicable as cooperative harmonic type systems, as well as FCC regulations requiring efficient use of frequency spectrum, place limits on the maximum allowable value for .DELTA. F. Other attempts to minimize quantization of the range measurement have altered the waveform of the difference frequency by superimposing on the modulating signal a slow wobbling of the transmitted frequency. However, such an approach is disadvantageous in that a larger bandwidth is required as compared to non-wobbling systems.
Still other attempts to minimize quantization have altered the difference frequency waveform by employing variable phase shifts, either linear, such as disclosed in U.S. Pat. No. 2,222,587 to R. C. Sanders, Jr. or non-linear, such as disclosed in U.S. Pat. No. 3,340,529 to D. Blitz, on either the transmitted or returned signal prior to deriving the difference signal.
In contrast, the processor of the present invention does not alter the difference frequency waveform but instead digitally processes the waveform in such a manner as to allow accurate range measurements to be made without the above described quantization limitation.